Radar system with phase based multi-target detection

ABSTRACT

A radar system includes a plurality of antennas and a controller. The plurality of antennas is configured to detect a reflected radar signal reflected by an object in a field-of-view of the system. Each antenna of the plurality of antennas is configured to output a detected signal indicative of the reflected radar signal detected by the antenna. The controller is configured to receive detected signals from the plurality of antennas, and determine if a target is present in the field-of-view based on the detected signals. The controller is also configured to determine if the target includes more than one object based on an analysis of phases of the detected signals.

TECHNICAL FIELD OF INVENTION

This disclosure generally relates to a radar system, and moreparticularly relates to a system that determines if a target includesmore than one object based on a comparison of phases of detected signalsfrom multiple receive antennas.

BACKGROUND OF INVENTION

Because of antenna size, system size, technology, and cost constraints,automotive radar sensors may have performance limitations with regard todiscriminating two objects that have similar position and Doppler shiftcharacteristics, or if one object has a substantially larger Radar CrossSection (RCS) than a second nearby object. Examples of two objects withsimilar range and Doppler shift reflection characteristics that typicalautomotive radar systems have difficulty discerning include: a slowlymoving pedestrian walking around stationary or slowly moving passengervehicle, a motor cycle traveling beside a tractor-trailer traveling inan adjacent lane at a similar range and range rate, and two passengercars moving close to each other on adjacent lanes with similar rangerates.

SUMMARY OF THE INVENTION

Automotive systems such as Autonomous Intelligent Cruise Control,Collision Warning and Mitigation, and Blind Spot Detection use a radarsensor to detect objects proximate to the vehicle. Reflected radarsignals are detected by an antenna array are typically converted to adiscreet base band, then transformed from the time-domain to thefrequency-domain where amplitude profiles indicative of each signaldetected by each receive antenna element are integrated non-coherently.Automotive radars often use this non-coherently integrated (NCI)amplitude profile for object detection to determine position (i.e.range) and Doppler parameters (i.e. range-rate) of detected objects withprofile magnitudes greater than a defined detection threshold. TheNCI-detection technique is preferred as it suppresses noise variation toavoid false alarms. While NCI may degrade information in the detectedsignals, it does so less than the noise variance as system noise is lesscorrelated across the antenna elements when compared to the reflectedradar signal. As such, NCI provides a net gain in signal-to-noise ratio.

However, NCI-detection techniques have performance limitations fordetection and discrimination of multiple objects that are near eachother. That is, if two objects have similar position and Dopplercharacteristics, and the reflection characteristics or Radar CrossSections (RCS) to the objects are significantly different, then thereflected radar signal from the object with the larger RCS may mask thereflected radar signal from target with smaller RCS, and thereby makessecond target identification and/or discrimination difficult. Examplesof this situation include a slowly moving pedestrian near a stationarypassenger vehicle, a motor cycle moving with nearly the same range andDoppler as a tractor-trailer type vehicle in an adjacent lane, and twopassenger vehicles moving close to each other on adjacent lanes withnearly the same range and Doppler.

Such a performance limitation could be improved by revised waveformparameter specification, and/or narrow beam antenna design. However,these options undesirably increase sensor size, cost, and signalprocessing complexity. Furthermore, it has been observed that some ofthe spectral parameters such as “spectrum beam width” are not a suitablefor post-processing technique to extract reliable information formultiple objects from the energy content under the broaden spectrum.U.S. patent application Ser. No. 14/277894 filed 15 May 2014 andentitled RADAR SYSTEM WITH IMPROVED MULTI-TARGET DESCRIMINATION byAlebel Arage Hassen, describes a detection system and method to improvethe detection and discrimination of multiple near targets that uses acomposite detection technique from a NCI-detection technique and an“or-logic” based single receive channel peaks detection technique.However, there is still a performance limitation to address and a desirefor a less computational intensive way to determine if a detected targetincludes or comprises more than one object.

The performance limitations of near targets detection and discriminationof the detection technique using amplitude spectrum peak detection andevaluation technique is solved by local phase spectrum evaluationtechnique. When multiple, near each other scattering centers of anobject or objects are present, reflected radar signals from eachscattering center interfere with each other differently at the variousreceive antenna elements depending on the relative position differenceof the various scattering centers with respect to the receive antennaposition. Relative position difference between scattering centers isexpressed in terms of relative phase difference between the reflectedradar signals from the various scattering centers, and it determinesinterference characteristics at the receive antenna. That means thatvarious antenna-array elements receive dissimilar interferencecharacteristics of reflected radar signals from these scattering centersdue to the fact that relative phase difference changes acrossdistributed antenna elements.

As it is a matter of interference between reflected radar signals fromdistinct scattering centers located in a close relative proximity toeach other, a phase difference of detected signals from each receiveantenna element can be evaluated in the frequency domain local to thesuperposed signal frequency bin. For example, a phase difference may becalculated between the first symmetrical (i.e. first higher and lower)frequency bins to the superposed signal detection frequency bin. Thephase difference converges to a minimum value (or zero) if detectedsignals are from a single point scattering center. This is becausesignal amplitude and phase spectrums are generally spread equally toneighboring frequency bins as far as the time domain signal is weightedby symmetrical window coefficients about the window maximum at thecenter. In case of reflected radar signal interference from multiplenearby scattering centers, these first symmetrical frequency bins shouldcontain different signal phase values as far as scattering centerspossess a relative position difference.

For antenna-array configurations, averaging of the phase differenceacross the antenna-array elements provides a robust phase differencevalue that can be used to distinguish a single point scattering centerfrom multiple near scattering centers. As discussed, there is also phasedifference variation across antenna-array elements due to the fact thatthe relative position difference of scattering centers are not equal fordistributed antenna-array configuration. Therefore, evaluating the slopeor standard deviation of the phase difference across antenna-arrayelements can also be employed to distinguish single scattering centerfrom multiple near scattering centers.

Note that a local phase spectrum evaluation technique by itself does notprovide parameter estimation of the scattering centers as it was thecase for the detection and discrimination techniques described by Hassen(appl. Ser. No. 14/277894). However, it is more sensitive to distinguisha reflection of a single scattering center from multiple near to eachother scattering centers. It can be then used as indicator to activatecontrolled parameter estimation techniques such as the amplitudespectrum peak detection and evaluation technique using an “or-logic”single channel detection technique as described by Alebel Arage Hassen(appl. Ser. No. 14/277894), or a complex spectrum evaluation techniqueusing space-time adaptive processing. Alternatively, this local phasespectrum evaluation technique can also be used to let the system defineRange-Doppler Near-Objects Detection zone (RDNOD-zone) about theNCI-detection if activation of parameter estimation technique is notrealistic. This will assist optimal usage of limited signal processingresources while enhancing identification of multiple near scatteringcenters.

It is recognized that since this is a phase difference evaluationtechnique as opposed to an absolute phase evaluation, it is lesssusceptible to mismatch between antenna-array elements as well as anytransients effects. However, it is recommended to apply this techniquefor detections only with adequate signal-to-noise ratio. Phase ingeneral is susceptible to noise, and results from phase differenceevaluation may not be reliable for detections with inadequatesignal-to-noise ratio.

The incapability of detected object classification by automotive radarsensor that uses amplitude spectrum peak detection and evaluationtechnique is improved by a local phase spectrum evaluation technique. Itwas observed that the standard deviation of the phase differencefluctuates over time if the scattering centers are in continuous motionand trigger relative position variance with time. As such, it isrecognized that the time domain variance (fluctuation) of the standarddeviation of the phase difference provides further information that canbe employed to distinctly classify or categorize various object groupsand their motion profiles, for example, to distinguish pedestrians,bicyclists, and vehicles moving at an angle relative to the host vehiclefrom vehicles traveling straight forward or longitudinally relative tothe host vehicle.

In accordance with one embodiment, a radar system is provided. Thesystem includes a plurality of antennas and a controller. The pluralityof antennas is configured to detect a reflected radar signal reflectedby an object in a field-of-view of the system. Each antenna of theplurality of antennas is configured to output a detected signalindicative of the reflected radar signal detected by the antenna. Thecontroller is configured to receive detected signals from the pluralityof antennas, and determine if a target is present in the field-of-viewbased on the detected signals. The controller is also configured todetermine if the target includes more than one object based on ananalysis of phases of the detected signals.

In another embodiment, a controller for a radar system is provided. Thecontroller includes a receiver and a processor. The receiver isconfigured to receive detected signals from a plurality of antennasconfigured to detect a reflected radar signal reflected by an object ina field-of-view of the system. Each antenna of the plurality of antennasis configured to output a detected signal indicative of the reflectedradar signal detected by each of the antenna. The processor isconfigured to receive the detected signals from the plurality ofantennas, determine if a target is present in the field-of-view based onthe detected signals, and determine if the target includes more than oneobject based on an analysis of phases of the detected signals.

In another embodiment, a method of operating a radar system is provided.The method includes the step of receiving detected signals from aplurality of antennas configured to detect a reflected radar signalreflected by an object in a field-of-view of the antennas. Each antennaof the plurality of antennas is configured to output a detected signalindicative of the reflected radar signal detected by each of theantenna. The method also includes the step of determining if a target ispresent in the field-of-view based on the detected signals. The methodalso includes the step of determining if the target includes more thanone object based on an analysis of phases of the detected signals.

Further features and advantages will appear more clearly on a reading ofthe following detailed description of the preferred embodiment, which isgiven by way of non-limiting example only and with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

The present invention will now be described, by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a diagram of a radar system in accordance with one embodiment;

FIG. 2 is a graph of signal present in the system of FIG. 1 inaccordance with one embodiment;

FIG. 3 is a graph of signal present in the system of FIG. 1 inaccordance with one embodiment;

FIG. 4 is a graph of data present in the system of FIG. 1 in accordancewith one embodiment;

FIG. 5 is a graph of data present in the system of FIG. 1 in accordancewith one embodiment; and

FIG. 6 is a flowchart of a method executed by the system of FIG. 1 inaccordance with one embodiment.

DETAILED DESCRIPTION

FIG. 1 illustrates a non-limiting example of a radar system, hereafterreferred to as the system 10. The system 10 includes an antenna array 12that may include a transmit-element 14, and an array of receiveelements, hereafter referred to as a plurality of antennas 16. It isrecognized that one or more of the antenna elements that make up theantenna array 12 could be used to both transmit a radar signal 18, andoutput a detected signal 30 indicative of reflected radar signals 20reflected by a first object 24A or a second object 24B in afield-of-view 22 of the system 10. The transmit-element 14 and theplurality of antennas 16 are illustrated as distinct elements in thisexample only to simplify the explanation of the system 10.

The system 10 may also include a controller 26 configured to output atransmit-signal 28 to the transmit-element 14, and configured to receivedetected signals 30 from each antenna, for example a first signal 30Afrom a first antenna 16A and a second signal 30B from a second antenna16B. Each of the detected signals 30 correspond to the reflected radarsignal 20 that was detected by one of the plurality of antennas 16. Thecontroller 26 may include a processor 27 such as a microprocessor,digital signal processor, or other control/signal conditioning circuitrysuch as analog and/or digital control circuitry including an applicationspecific integrated circuit (ASIC) for processing data, as should beevident to those in the art. The controller 26 may include memory (notshown), including non-volatile memory, such as electrically erasableprogrammable read-only memory (EEPROM) for storing one or more routines,thresholds and captured data. The one or more routines may be executedby the processor 27 to perform steps for determining if the detectedsignals 30 received by the controller 26 indicate the presence of thefirst object 24A or the second object 24B, as described herein.

To meet customer specified angular-resolution requirements of automotiveradar systems, such systems often use antennas that have relativelynarrow transmit and receive beam-widths to scan a field-of-view forobjects. In this non-limiting example, the transmit-element 14 radiatesor emits the radar signal 18 toward the first object 24A and/or thesecond object 24B in a field-of-view 22, and the plurality of antennas16 each detect a reflected radar signal reflected by the first object24A and/or the second object 24B in the field-of-view 22 of the system10. Characteristics of the reflected radar signal 20 depend on abackscatter property or radar cross section (RCS) of the first object24A or the second object 24B. The characteristics also depend ondistance, direction, and relative motion of the first object 24A and/orthe second object 24B relative to the antenna array 12, which influencesthe Doppler shift of the reflected radar signal 20. Depending on thesignal waveform and the modulation system used, the controller 26 maytransform the time domain signals (the detected signals 30) to thefrequency domain so, for example, the spectrums can be combined using,for example, non-coherent integration (NCI). Some automotive radarsystems use this non-coherently integrated spectral data as the basisfor object detection, and evaluate the spectral data to determine theposition and Doppler parameter estimates that have higher spectralmagnitude than a defined detection threshold. NCI is generally preferredto suppress noise induced variation and thereby keep noise induced falsealarm rates to a minimum.

If multiple objects are present in the field-of-view 22, the reflectedradar signal 20 may interfere with each other depending on the relativeposition and/or range rate difference between the objects with respectto the receive antennas (the plurality of antennas 16). A relativeposition difference between the first object 24A and the second object24B is illustrated as Arx and Ary and may be exhibited in terms of arelative phase difference between the reflected radar signal 20 detectedby the antennas 16 from these scattering centers. That may cause thedetected signals 30 to exhibit dissimilar interference characteristicsfor the signals from the scattering centers of the objects due to thefact that the relative phase difference changes across the plurality ofantennas 16. This leads to different range profiles and Doppler profilesacross the plurality of antennas 16, and increases the probability toget instantaneous multiple spectral peaks and nulls if the detectionstrategy is based on an ‘or-logic’ comparison of the single-channel orindividual signals. Depending on the number of elements in the pluralityof antennas 16, this detection concept improves detection anddiscrimination of nearby scattering centers. By contrast, NCI baseddetection suppresses the position difference effect of scatteringcenters by averaging out the spectrum difference across the detectedsignals 30, which makes nearby scattering center resolution anddiscrimination more difficult.

Applicant's prior system described in U.S. patent application Ser. No.14/277,894 filed 15 May 2014 applies a composite detection strategybased on NCI spectrum together with a single receive channel spectrumanalysis using ‘or logic’ in order to improve automotive radar range,range rate, and angle measurement resolution, and enhance systemperformance for near targets discrimination, target imaging, and lateralrange rate estimation. A time delay between transmitted and receivedsignals as well as the frequency shift due to Doppler effect is used tocompute radial distance (e.g. r1 or r2 in FIG. 1) and relative velocityof a detected object, e.g. the first object 24A or the second object24B, respectively. The received signal-phase differences of the detectedsignals 30 are used to estimate the angle (direction) of a detectedobject by applying various angle finding techniques or algorithms suchas Monopulse, digital beam forming, or super-resolution.

Object detection by the prior system may be first done in theRange-Doppler (RD) domain after applying a 2D-FFT algorithm to thedetected signals 30, and then integrating the resulting range-Dopplerspectrums non-coherently. Local maxima of the resultant NCI RD-image andtheir immediate adjacent neighboring spectrums are used and processed todetect object and determine its corresponding RD-coordinates includinglateral and longitudinal position of the object after applying thedesired angle finding algorithm on the detection raw spectral data.

In certain situations, multiple objects could have nearly the same rangeand Doppler parameters. The range and Doppler differences between theseobjects can be smaller than RD-measurement resolution, which is mainlypredetermined from signal waveform parameters like sweeping frequencyand dwell time. As result, these objects can appear as one local maximaof the NCI RD-image, and their discrimination will only depend on angleif they possess lateral span that is consistent with measurementresolution of the applied angle finding technique (i.e. antenna patternbeam width, configuration, and angle evaluation algorithm). That means,for relatively nearby targets with inadequate Doppler, longitudinal, andlateral separations, the performance of multiple targets discriminationis limited for NCI only RD-image based detection strategy.

For a specific radar system design, such a limitation in resolution anddiscrimination performance can be improved significantly if thedetections strategy evaluates not only a composite NCI RD-image, butalso each of the antenna signals on an individual basis, i.e. singlereceive channel RD-images. As described above, signals from two nearbyscattering centers of an object or objects may interfere at the receiveantenna element depending on signals relative phase difference betweenthese scattering centers. This relative phase difference is a functionof the lateral and longitudinal range separation (e.g. Arx, Ary) betweenthese two scattering centers, and may not be equal across the pluralityof antennas 16. This is especially true for automotive radar thatoperates at millimeter wave, 3.92 mm for example, which is much smallerthan in the real world expected position difference between scatteringcenters. As a result, spectrums of the signals interference from thesescattering centers should possess dissimilar profile between receiveantenna-array elements, and show peaks and nulls at different range andDoppler frequencies for different antenna-array elements. An improvedway to determine if the target 24 includes or is made up of more thanone object, e.g. the first object 24A and the second object 24B.

The system 10 described herein may be used as part of an automateddriving system that controls various aspects of the vehicle such asvehicle speed and/or automated braking. If a radar system installed in ahost vehicle was unable to detect a nearby object such as a motorcycledirectly forward of the host vehicle by discriminating the motorcyclefrom a larger, further away object detected by NCI, a semi-trailer in atravel lane adjacent the lane of the host vehicle, the speed controlsystem may undesirably accelerate the host vehicle toward themotorcycle. That is, the larger signal reflected from the trailer maymask the smaller signal reflected from the motorcycle if they are nearto each other in range and/or have similar range rates. In such cases,the NCI detects only one peak within a broad spectrum. Since the twoobjects are in adjacent lanes, the system 10 may determine only oneangle tending to be for the larger signal and not be able todiscriminate the angle of one object from the angle of the other,especially at longer ranges due to limited angular resolution of theangle finding technique used. This is an example of why near targetdiscrimination on the range profiles and/or Doppler profiles orrange-Doppler images is advantageous to reliably track objects the hostvehicle lane.

Referring again to FIG. 1, a non-limiting example of the system 10includes a plurality of antennas 16 configured to detect a reflectedradar signal 20 reflected by an object (24A, 24B) in a field-of-view 22of the system 10, wherein each antenna (e.g. the first antenna 16A andthe second antenna 16B) of the plurality of antennas 16 is configured tooutput a detected signal (e.g. the first signal 30A and the secondsignal 30B) indicative of the reflected radar signal 20 detected by eachof the antenna 16A, 16B, . . . . The controller 26 is generallyconfigured to receive the detected signals 30 from the plurality ofantennas 16, determine if a target 24 is present in the field-of-view 22based on the detected signals 30, and determine if the target 24includes more than one object (e.g. the first object 24A and the secondobject 24B) based on an analysis of phases of the detected signals 30.

The controller 26 may include a receiver 29 configured to receive anantenna signal (e.g. the first signal 30A and the second signal 30B)from each antenna (e.g. the first antenna 16A and the second antenna16B) corresponding to the reflected radar signal 20 that was detected byeach of the plurality of antennas 16. The controller 26 may include amixer (not shown) and a local oscillator (not shown) in order todemodulate the detected signals 30. The mixer and the local oscillatormay be part of the receiver 29.

Radar signals reflected by two nearby scattering centers of a target ortargets formed of multiple objects interfere with each other to somedegree at the antennas 16. The degree of interference depends on arelative phase difference between the various reflected radar signalsfrom each object. This relative phase difference is a function of thelateral and longitudinal range separation between the two scatteringcenters, and cannot be equal across all of the antennas 16. As such, thephase spectrums of the interfered signals have different profiles acrossthe receive antenna-array elements (the antennas 16). Amplitudespectrums of different antenna-array elements can show peaks and nullsat different frequencies (i.e. ranges) depending on the relativeposition difference-to-wavelength ratio. Since some automotive radarsystems operate at micrometer and millimeter wavelengths, 12.5 mm and3.92 mm for example, the relationship of “relative positiondifference-to-wavelength ratio” makes the spectrum profile diversityacross antenna-array elements relatively dynamic and sensitive todiscriminate on-road near scattering centers of an object or objects.This sensitivity also depends on number of antenna-array elements, whichcreates opportunities to get multiple instantaneous peaks at multiplefrequency bins, and thereby increases the probability of detection anddiscrimination of near scattering centers using single receive channeldetection technique when compared to the NCI amplitude spectrum peakdetection techniques. NCI could be used to average out spectrumdiversity effects of the variance of relative phase difference acrossreceive antenna-array elements, and thereby degrade the detection and/ordiscrimination of a second nearby scattering center.

In cases where scattering centers position constellations result inuniform constructive interference across all receive antenna-arrayelements, the opportunity of receiving multiple instantaneous peaks atmultiple frequency bins diminishes as the local amplitude spectrums ofall receive antenna-array elements uniformly broadens and makes peaks atone and the same frequency bin only. This limits the performanceimprovement of near scattering centers detection and discriminationusing amplitude spectrum peak evaluation technique even with the“or-logic” single channel detection technique. Specially, the degree ofthe performance limitation is significant for radars with small numberof antenna-array elements as compared to radars with a greater numbersof antenna-array elements. Such a performance limitation is also oftenthe case for near scattering centers with significant RCS difference.For example, a pedestrian near to an automobile, or motorcycle near to atractor-trailer could experience up to 30 dBsm RCS difference. Thespectrum of the larger target can mask the spectrum of the smallertarget for all receive antenna-array elements, and makes amplitudespectrum peaks detection technique ineffective.

Typically, complete extraction of information from signals reflected byscattering centers requires a complex spectrum evaluation technique.Since the relative position difference between scattering centers isembedded in the superposed signal phase term, phase spectrum evaluationtechnique should still provide information about presence of nearscattering centers, and overcome performance limitations by theamplitude spectrum peak detection technique for scenarios discussedherein.

In addition, a typical automotive radar sensor has limited capability toclassify or categorize on road targets to distinguish, for example, apedestrian from a vehicle by tracking the micro Doppler effect ofpedestrian's motion. The pedestrian's micro Doppler detection depends onamplitude spectrum peak detection technique on the Doppler frequencydomain. As in the above sections discussed, the performance limitationof amplitude spectrum peak detection technique due to signalinterferences of multiple scattering centers degrades also the microDoppler effect detection and then the tracker's limited capability toclassifying pedestrian from objects like a vehicle.

The local phase spectrum evaluation technique proposed herein reinforcesthe radar tracking capability of classifying or categorizing targets byevaluating fluctuation of the phase difference with time. Depending onradar object geometry and motion profile, the phase differencefluctuates in time domain as far as scattering centers are in continuousmotion and trigger relative position variance with time. Therefore, thetime domain variance of the standard deviation of the phase differenceprovides further information that can be employed to classify from radardetected on-road objects in to various categories.

FIGS. 2 and 3 are non-limiting examples of a graph 200 and a graph 300that illustrate examples of data stored in the controller 26 of thesystem 10. The data in FIG. 2 corresponds to a reflected radar signalfrom a single object with a radar cross section (RCS) comparable to asingle passenger vehicle. In contrast, the data in FIG. 3 corresponds toa reflected radar signal from a two close together objects with a radarcross section (RCS) comparable to two passenger vehicles or twoscattering centers from the rear and the front of a single passengervehicle.

The detected signals 30 are typically time-domain signals that thecontroller 26 samples and performs a frequency transformation (e.g. aFourier transform) to generate a frequency profile 32 of each of thedetected signals, e.g. the first signal 30A and the second signal 30B.FIGS. 2 and 3 illustrate the amplitude portion 34 of the frequencyprofiles 32 arising from the frequency transformation. Those in the artwill recognize that a frequency transformation of radar reflectionsarising from certain types of emitted radar signals will indicate rangeto a target. Those in the art will also recognize that frequencytransformations may also generate phase information, see FIGS. 4 and 5,which are discussed in more detail below. In both cases (FIGS. 2 and 3),the amplitude portion 34 does not appear to be particularly useful todetermine if the target 24, which is located at about forty-one meters(41 m) of range and which corresponds to frequency bin #28, is a singlepoint reflection (e.g. only the first object 24A), or multi-pointreflection (e.g. the first object 24A and the second object 24B).

Accordingly, the controller is advantageously configured to determinethe frequency profiles 32 of each of the detected signals 30. Asdiscussed above, the frequency profile for each of the detected signals30 includes both an amplitude portion 34 as illustrated in FIGS. 2 and3, and includes a phase portion 36 (FIGS. 4 and 5). Each of thefrequency profiles 32 includes a plurality of amplitude values and phasevalues associated with frequency bins 38 that correspond to an amplitudesample and phase sample of a particular frequency profile at aparticular frequency. As will be recognized by those in the art, thefrequency bins 38 correspond to a range to a potential target, and theamplitude of the frequency profiles 32 is an indication of the amount ofradar signal reflected at a particular range. As such, if the amplitudeportion 34 of the frequency profile is relatively high, greater than 60dB for example, it is an indication that a target is present at or neara range or distance that corresponds to the frequency bin with thegreatest value of the amplitude portion 34. In FIGS. 2 and 3 thegreatest value of the amplitude portion is at frequency bin #28 whichcorresponds to about forty-one meters (41 m).

In one embodiment, the frequency profiles 32 may be characterized as arange profile based on a frequency transformation (e.g. Fouriertransform) of time-domain samples of the detected signals from all ofthe antennas. Alternatively, the frequency profiles 32 may becharacterized as a Doppler profile based on a frequency transformationof a time-domain samples of the detected signals from all antennas.Which alternative is used is dependent on the modulation used for theradar signal 18, for example frequency modulated continuous wave (FMCW),continuous wave (CW), or Pulse-Doppler. All of these modulations schemesprovide a time domain signal that can be time sampled and transformedinto the frequency domain. What differs is what the frequency profilerepresents.

For example, a system that uses FMCW waveform with adequate number ofantenna array elements may perform a 3D-Fourier transformation invarious orders. The first time sample data transformation to frequencydomain is to get range profile per chirp-pulse. For a given rangefrequency bin, it performs the second Fourier-transformation acrossmultiple chirp-pulses in order to get the Doppler-profile. For a givenrange-Doppler frequency bin, it performs the thirdFourier-transformation across antenna array elements to get anangle-profile (known as Digital-Beam-forming). It is noted that thiskind of 3D-frequency transformation order to determine range, Doppler,and angle profiles is an example that can be also performed in differentorders depending on the complexity for the intended applications.

Alternatively, a system that uses FMCW waveform with only two or threeantenna array elements may be processed using a 1D-Fouriertransformation only to transfer the time sample data to frequency domainusing a Fourier-transformation, and then build a so called Doppler-rangeplane and applying a matching technique between chirps per antennaelement. When an intersection between chirp frequencies domain signalsis found, target detections are determined with a Doppler-range index atthe intersection coordinate. After this is done for all antennaelements, a Monopulse technique (i.e. amplitude and phase comparisontechnique between antenna elements) is applied in order to get the angleof the detected target. For this technique, another frequencytransformation stage is unnecessary.

Another alternative is to use a Pulse-Doppler waveform, which doesn'trequire performing Fourier-transformation on time sampled data to getrange profile. Instead, so called range gates are defined as a functionof sampling sequences, which immediately starts after a single pulse iscompletely transmitted. For example, range gate 1=ts1/(2C), range gate2=ts2/(2C), . . . range gate N=tsN/(2C), where ts1, ts2, tsN are 1^(st),2^(nd), . . . N^(th) sampling time after a single pulse is transmitted.This is performed repeatedly for a number of successive pulses. For agiven range gate, it performs a Fourier-transformation on time sampledata across number of pulses to determine the Doppler profile. Sincethis is done for each of antenna array elements, depending on theimplemented antenna technique, it can apply different angle findingtechniques (including Monopulse, digital beam forming, . . . ) to getthe angle of the detected target in the specific range bin and Dopplerbin. That is, a Fourier transformation is performed to get a Dopplerfrequency profile as well as angle profile if the implemented antennatechnique requires performing digital beam forming.

Radar operating with CW waveform (e.g. Police radar), does detecttarget's with the Doppler-profile. It is performingFourier-transformation on time sample data to get the Doppler-profile.There is no range profile. So, as can be seen in the description above,all radars, regardless their waveform, transform the time sample data tofrequency domain, and determine target's frequency profile regardlesswhat it represents (range or Doppler or angle profile).

FIG. 4 illustrates a graph 400 of phase values 40 associated withselected range bins (#27 and #29) for each of the detected signals 30from each of the antennas 16 labeled in this illustration as the receivechannels 42. For example, the first signal 30A from the first antenna16A is processed to determine an amplitude value and a phase value foreach of the sampled frequencies associated with the frequency bins 38a.k.a. range bins. Then one of the range bins is designated as areference range bin 44 (#28 in this example) because it is associatedwith a maximum amplitude value 46. The maximum amplitude value 46 may beselected based on the maximum value of a combination of all the detectedsignals 30 by, for example, a non-coherent integration (NCI) of thefrequency profile's 32.

FIG. 5 illustrates a graph 500 of phase differences 50 of each frequencyprofile at two distinct frequencies, phase bins #27 and #29 in thisexample. That is, each of the phase differences 50 is the difference inphase values 40 in two of the frequency bins 38 for one of the frequencyprofiles 32. The single object plot 52 derived from the graph 200 (FIG.2) shows that there is little change in the phase differences 50 acrossthe receive channels 42, while the two object plot 54 derived from thegraph 300 (FIG. 3) show that there is a perceivable change in the phasedifferences 50 across the receive channels 42.

In summary, the frequency profiles 32 are characterized by values (theamplitude portion 34 and the phase portion 36) stored in an array offrequency bins 38. The phase differences 50 is determined based on adifference between a first phase value 40A associated with a firstfrequency bin 44A (#27) of the array, and a second phase value 40Bassociated with a second frequency bin 44B (#29) of the array. Thereference frequency bin 44, which is used to select the first frequencybin 44A and the second frequency bin 44B, is associated with a maximumamplitude value 46. By way of example and not limitation, the firstfrequency bin 44A is adjacent to the reference frequency bin 44 andassociated with a lower frequency than the reference frequency bin 44,and the second frequency bin 44B is adjacent to the reference frequencybin 44 and associated with a higher frequency than the referencefrequency bin 44.

In general, the reference frequency bin 44 should have sufficient signalstrength so noise is not a substantial problem to determine the phasedifference between any of the of the frequency bins 44A, 44 and 44B.

Referring again to FIG. 5, it is evident that the two object plot 54 issloped, while the single object plot 52 is relatively flat, i.e. is notsloped. Accordingly, the controller 26 may be configured to determine aphase slope 56 based on a trend or trend line in the phase differences50 versus a relative position of each of the antennas 16 that determinesthe two object plot 54. The phase slope 56 may be determine using anynumber of method known to those in the mathematical arts, a leastsquares fit to the well-known equation y=mx+b for example, where m isthe slope. The controller 26 may also be configured to indicate that thetarget 24 includes more than one object, e.g. the firs object 24A andthe second object 24B, if the phase slope 56 has a magnitude greaterthan a slope threshold 58. It is recognized that the phase slope 56illustrated would generally be characterized as a negative slope, butpositive slopes are possible for other example targets. That is why themagnitude or steepness of the phase slope is coppered to the slopethreshold which is also considered in terms of absolute value and not asigned value. The value selected for the slope threshold 58 may beempirically determined through laboratory and/or field testing.

Alternatively, and likely less computationally intensive, the controller26 may be configured to indicate that the target 24 includes more thanone object if the phase difference variation 60 is greater than avariation threshold (not shown but understood to be a predeterminedvalue). The phase difference variation 60 may be determined by, forexample, calculating a standard deviation of the phase differences 50 ofthe two object plot 54. If the standard deviation is greater than thepredetermine value of the variation threshold, then that is anindication that the target 24 includes more than one object. The valueselected for the variation threshold may be empirically determinedthrough laboratory and/or field testing.

FIG. 6 illustrates a non-limiting example of a method 600 of operating aradar system (the system 10). In particular, the method 600 is directedto determining if a target 24 detected by the system 10 includes or ismade up of more than a single object based on an analysis of phases(e.g. the phase values 40) of the detected signals 30.

Step 610, RECEIVE DETECTED SIGNALS, may include a controller 26receiving the detected signals 30 from a plurality of antennas 16 thatare configured to detect a reflected radar signal 20 reflected by anobject (24A, 24B) in a field-of-view 22 of the antennas 16. Each antenna(16A, 16B) of the plurality of antennas is configured to output adetected signal (30A, 30B) indicative of the reflected radar signaldetected by each of the antennas 16.

Step 620, DETERMINE TARGET PRESENCE, may include determining if a target24 is present in the field-of-view 22 based on the detected signals 30.The target 24 may be detected by determining the frequency profiles 32of each detected signals 30 by applying frequency transformation (e.g.Fourier transform) to the time-domain signals from the antennas 16.Frequency transformations typically provide sampled values of thefrequency spectrum arising from the frequency transformation and includean amplitude portion 34 and a phase portion 36.

Step 630, DETERMINE REFERENCE FREQUENCY, may include detecting a maximumamplitude value 46 of the amplitude portion 34 of the frequency profiles32. The maximum amplitude value 46 may be a maximum or peak value fromsome composite of the frequency profiles 32 such as a non-coherentintegration (NCI) of the frequency profiles 32. Each frequency profilemay be characterized as a range profile or a Doppler profile based on afrequency transformation of a time-domain sample of the detected signalfrom one antenna, as previously described. The frequency profiles 32 maybe sampled to generate values for frequency bins 38 that representstorage locations for the values. A reference frequency bin 44 isassociated with a maximum amplitude value.

Step 640, DETERMINE FIRST FREQUENCY AND SECOND FREQUENCY, may includedetermining which of the frequency bins contains an amplitude valuegreater that some predetermine threshold. In order to calculate phasedifferences 50 a first frequency bin 44A and a second frequency bin 44Bmay be designated. The first frequency bin 44A may be adjacent to thereference frequency bin 44 and associated with a lower frequency thanthe reference frequency bin 44, and the second frequency bin 44B may beadjacent to the reference frequency bin 44 and associated with a higherfrequency than the reference frequency bin 44. Alternatively, if thesignal strength of either of the bins adjacent to the referencefrequency bin 44 is too low or weak, the reference frequency bin 44 maybe designated either the first frequency bin 44A or the second frequencybin 44B.

Step 650, DETERMINE PHASE DIFFERENCES, may include determining a phasedifferences 50 of each of the frequency profiles 32 at two distinctfrequencies, e.g. frequencies associated with the first frequency bin44A and the second frequency bin 44B. Each frequency profile ischaracterized by values stored in an array of frequency bins 38, and inthis non-limiting example the phase difference is determined based on adifference between a first phase value 40A associated with the firstfrequency bin 44A of the array of receive channels 42, and a secondphase value 40B associated with a second frequency bin 44B of the arrayof receive channels 42.

Steps 660 and 670 may both be performed, bit it is more likely that onlyone or the other may be performed. The showing of both steps 660 and 670in method 600 should not be construed to mean that it is required thatboth steps are performed. If either or both of the tests performed bysteps 660 and 670 result in the affirmative (YES), the method 600proceeds to step 680.

Step 660, PHASE DIFFERENCE VARIATION>VARIATION THRESHOLD?, may includedetermining a phase difference variation 60 based on changes in thephase differences 50 versus a relative position of each antenna, whichcorresponds to the numbering of the receive channels 42, and determiningif the phase difference variation 60 is greater than a variationthreshold.

Step 670, PHASE SLOPE>SLOPE THRESHOLD?, may include determining a phaseslope 56 based on a trend in the phase differences 50 versus a relativeposition of each antenna, and determining if the phase slope has amagnitude greater than a slope threshold.

Step 680, INDICATE TARGET INCLUDES MORE THAN ONE OBJECT, may include thecontroller 26 indicating that the target 24 includes more than oneobject if the phase difference variation 60 is greater than a variationthreshold, and/or indicating that the target 24 includes more than oneobject if the phase slope 56 has a magnitude greater than a slopethreshold 58. The indication that the target 24 includes more than oneobject may, for example, result in the controller initiating othersoftware routines to further examine signals from the antennas for thepurpose of classifying or categorizing the multiple objects thatconstitute the target 24, or illuminating an indicator to notify anoperator of the vehicle that a pedestrian proximate to the vehicle hasbeen detected.

Accordingly, a radar system (the system 10), a controller 26 for thesystem 10 and a method 600 of operating the system 10 is provided. Theseall include an improved way to determine if more than one target ispresent in the field-of-view 22 where prior attempts may have detectedonly one target. The standard deviation of the phase difference acrossantenna array elements indicates a single scattering center or multiplenear scattering centers. If multiple near scattering centers areindicated, other near targets detection and discrimination techniquessuch as the single channel peak detection technique and the space timeprocessing technique can be activated. The indication can also be usedto guide the system 10 to define a Range-Doppler Near Objects Detectionzone (RDNOD-zone) of the near scattering centers around theNCI-detection RD-coordinate if there is signal processing resourceconstraint or if the radar has a relatively small number ofantenna-array elements (the antennas 16).

The time-domain fluctuation of standard deviation of the phasedifference can also be used to classify radar on-road object categories.This object classifier can be used by a tracker module to make objectguided parameter prediction and enhance object tracking performance.This is particularly relevant for reliably tracking laterally movingobjects such as a pedestrian and a bicyclist as the radar sensor isincapable of direct lateral rate measurement. In general, the benefitsof the system described herein contributes to, but is not limited to:Enhanced Near Targets identification and Discrimination; Automotiveradar Target extent measurement (or imaging); Cross traffic detectionand reliable tracking as result of targets discrimination andclassification; and optimal usage of limited signal processing resourceswhile improving the performance of near targets identification,discrimination, and tracking. This technique described herein isapplicable in many configurations of automotive radar sensor products,and implementation is straight forward with modest increase in signalprocessing throughput and memory.

While this invention has been described in terms of the preferredembodiments thereof, it is not intended to be so limited, but ratheronly to the extent set forth in the claims that follow.

We claim:
 1. A radar system comprising: a plurality of antennasconfigured to detect a reflected radar signal reflected by an object ina field-of-view of the system, wherein each antenna of the plurality ofantennas is configured to output a detected signal indicative of thereflected radar signal detected by each of the antenna; and a controllerconfigured to receive the detected signals from the plurality ofantennas, determine if a target is present in the field-of-view based onthe detected signals, and determine if the target includes more than oneobject based on an analysis of phases of the detected signals.
 2. Thesystem in accordance with claim 1, wherein the controller is furtherconfigured to determine a frequency profile of each detected signal, anddetermine a phase difference of each frequency profile at two distinctfrequencies.
 3. The system in accordance with claim 2, wherein thefrequency profile is characterized as a range profile based on afrequency transformation of a time-domain sample of the detected signalfrom one antenna.
 4. The system in accordance with claim 2, wherein thefrequency profile is characterized as a Doppler profile based on afrequency transformation of a time-domain sample of the detected signalfrom one antenna.
 5. The system in accordance with claim 2, wherein thefrequency profile is characterized by values stored in an array offrequency bins, and the phase difference is determined based on adifference between a first phase value associated with a first frequencybin of the array, and a second phase value associated with a secondfrequency bin of the array.
 6. The system in accordance with claim 5,wherein a reference frequency bin is associated with a maximum amplitudevalue, the first frequency bin is adjacent to the reference frequencybin and associated with a lower frequency than the reference frequencybin, and the second frequency bin is adjacent to the reference frequencybin and associated with a higher frequency than the reference frequencybin.
 7. The system in accordance with claim 2, wherein the controller isfurther configured to determine a phase slope based on a trend in thephase difference versus a relative position of each antenna, andindicate that the target includes more than one object if the phaseslope has a magnitude greater than a slope threshold.
 8. The system inaccordance with claim 2, wherein the controller is further configured todetermine a phase variation based on changes in the phase differenceversus a relative position of each antenna, and indicate that the targetincludes more than one object if the phase variation is greater than avariation threshold.
 9. A controller for a radar system, said controllercomprising: a receiver configured to receive detected signals from aplurality of antennas configured to detect a reflected radar signalreflected by an object in a field-of-view of the system, wherein eachantenna of the plurality of antennas is configured to output a detectedsignal indicative of the reflected radar signal detected by each of theantenna; and a processor configured to receive the detected signals fromthe plurality of antennas, determine if a target is present in thefield-of-view based on the detected signals, and determine if the targetincludes more than one object based on an analysis of phases of thedetected signals.
 10. A method of operating a radar system comprising:receiving detected signals from a plurality of antennas configured todetect a reflected radar signal reflected by an object in afield-of-view of the antennas, wherein each antenna of the plurality ofantennas is configured to output a detected signal indicative of thereflected radar signal detected by each of the antenna; and determiningif a target is present in the field-of-view based on the detectedsignals; and determining if the target includes more than one objectbased on an analysis of phases of the detected signals.
 11. The methodin accordance with claim 10, wherein the method includes determining afrequency profile of each detected signal, and determining a phasedifference of each frequency profile at two distinct frequencies. 12.The method in accordance with claim 11, wherein the frequency profile ischaracterized as a range profile based on a frequency transformation ofa time-domain sample of the detected signal from one antenna.
 13. Themethod in accordance with claim 11, wherein the frequency profile ischaracterized as a Doppler profile based on a frequency transformationof a time-domain sample of the detected signal from one antenna.
 14. Themethod in accordance with claim 11, wherein the frequency profile ischaracterized by values stored in an array of frequency bins, and thephase difference is determined based on a difference between a firstphase value associated with a first frequency bin of the array, and asecond phase value associated with a second frequency bin of the array.15. The method in accordance with claim 14, wherein a referencefrequency bin is associated with a maximum amplitude value, the firstfrequency bin is adjacent to the reference frequency bin and associatedwith a lower frequency than the reference frequency bin, and the secondfrequency bin is adjacent to the reference frequency bin and associatedwith a higher frequency than the reference frequency bin.
 16. The methodin accordance with claim 11, wherein the method includes determining aphase slope based on a trend in the phase difference versus a relativeposition of each antenna, and indicating that the target includes morethan one object if the phase slope has a magnitude greater than a slopethreshold.
 17. The method in accordance with claim 11, wherein themethod includes determining a phase variation based on changes in thephase difference versus a relative position of each antenna, andindicating that the target includes more than one object if the phasevariation is greater than a variation threshold.